The invention relates to a startup circuit for a switched power supply having an undervoltage monitor and pulse width modulator as a control portion that has at least one immediate start input and one inable input.
Switched power supplies usually have a monitoring unit that disconnects the power supply via a control and regulating unit given downward movement of a specific value of the output voltage. In order to enable the startup operation of the power supply, an undervoltage monitoring unit must be disabled simultaneously with the startup instruction.
This usually occures in such manner that a one-shot multivibrator is started with the startup instruction. The one-shot multivibrator disables the undervoltage monitoring unit and thus enables the control and regulating unit of the power supply. After the lapse of a time defined by the wiring of the one-shot multivibrator, the undervoltage monitoring unit is reconnected. In addition to the high susceptibility of one-shot multivibrators to disruption, the disadvantage of this principle is that the time constants ofthe one-shot multivibrator and soft start circuit must be very precisely matched to one another in order to guarantee a reliable startup of the power supply.